The present invention relates to a wheel assembly for a vehicle, and more particularly to a wheel assembly for a material handling vehicle such as a pallet truck.
Vehicles, such as material handling vehicles (e.g., pallet trucks, reach trucks, counterbalance trucks, tow tractors, order pickers, etc.), utility carts, wagons, etc. incorporate wheels in a variety of roles, such as a drive wheel, a steering wheel, a support wheel, or some combination thereof. In some configurations, the wheel assembly includes a caster wheel. All of the wheels will wear over time and will eventually require maintenance to repair or replace the wheel.
In the material handling industry, increased load carried by the wheels, smaller wheel diameters, and higher rotational velocities of the wheels tend to exacerbate the wear, further impacting the useful life of a wheel.
A material handling vehicle, and in particular, a pallet truck is often equipped with a main drive wheel and one or more additional wheels. These additional wheels, which may be casters, are included, for example, to enhance handling and maneuverability. Although casters behave well when properly maintained, it can be possible for the caster to fall out of adjustment as the drive wheel wears. Adjusting casters can be a time consuming process.
Traditional casters require periodic adjustment to compensate for drive wheel wear. This adjustment is normally done by adding or removing shims between the caster and the vehicle to raise or lower the caster. The adjustment process can be labor intensive. In certain cases, to adjust the caster, the vehicle must be elevated and the caster must be removed before shims can be added or removed.
More advanced casters have adjustment screws that can raise or lower the caster to facilitate periodic adjustments. The adjustment screws can be accessed from the side on some designs and from the top on others. In this case, the casters can be adjusted without removing the caster but the adjustment point is under the vehicle. Top adjust casters provide an easier access point but require a hole in the operator floor.
Fundamentally, a disadvantage of current caster systems for material handling vehicles is the necessity for periodic adjustment. Therefore, a need exists for an improved wheel assembly for a vehicle that reduces the frequency of periodic adjustments of the caster wheels. Furthermore, a need exists for a means for providing a definitive indication to assist maintenance technicians in determining when drive wheel or caster wheel repair or replacement is required.
In another aspect, a material handling vehicle may include one or more spring-loaded or sprung caster wheels. For example, sprung caster wheels may be installed on end rider or center rider pallet truck including one or more lifting forks to provide a more stable platform for the vehicle. Whereas a drive tire or a load wheel may bear the majority of a load carried by a material handling vehicle, a sprung caster wheel may provide a restoring force during turning or cornering maneuvers. The restoring force provided by the sprung casters may be useful to minimize vehicle roll or to improve the stability of a load carried by the forks.
Generally, sprung caster wheels for material handling vehicles may be adjusted to provide a set preload force, such as about 1.1 kilonewtons (kN) or about 250 lb-force (lbf). However, as one or more of the drive wheel, load wheel or caster wheels wear during operation of the material handling vehicle, the force may build linearly (or non-linearly) as the deflection across the caster wheel increases. In the example case of a pallet truck with a drive wheel and a pair of flanking sprung caster wheels, as the tire of the drive wheel wears, the deflection across the casters wheels may increase. This may cause the caster wheels to bear a greater load which in turn may require the force on the caster wheels to be adjusted, for example, to maintain one or more performance characteristics of the material handling vehicle. As described above, adjusting caster wheels may be a time consuming process depending on the location of the caster wheels and the method by which the caster wheel are accessed or adjusted.
In a related aspect, for a material handling vehicle with two or more sprung caster wheels, it may be useful to provide a torsion bar as described in U.S. Pat. No. 7,770,904 (hereinafter, the '904 patent). The '904 patent describes that a material handling vehicle may include a pair of swivel casters mounted with respective conventional springs and coupled by a torsion bar. However, for at least the reasons described above, the use of caster wheels with conventional springs to provide a restoring force may have several drawbacks. Accordingly, a need exists to provide a system that may provide roll resistance, for example, to stabilize the vehicle, while also reducing the frequency with which maintenance must occur to adjust caster wheels.